The problem of removing foreign material and imperfect soybean seed from good soybean seed has existed since soybeans have been grown as a crop. As this problem was recognized, the spiral separator was adapted for processing soybean seed.
The spiral separator is a machine used to process agricultrual seed. It was originally designed to remove vetch seed from wheat seed. The machine is currently being used to process soybean seed. The machine separates whole soybean seed from contaminants such as corn, split soybean seed, moldy soybeans and impurities that do not roll as freely as whole soybeans. The machine separates seeds based on shape, density, and degree of roundness or ability to roll.
The spiral separator consists of one or more inner sheet metal flights spirally wound around a central axis. Seed is discharged onto the upper end of these flights. Nearly spherical seed travels at a faster rate than less round seed. Seed momentum increases until the round seeds roll off the edge of the inner spirals and are caught in the outer spiral. The good seed in the outer spiral and the contaminants which remain on the inner spirals are discharged separately from the machine. The rate of incline in the spirals and the banking angle of the flights are predetermined and fixed by the manufacturer.
The only adjustment on commercially existing spiral separators is rate of feed of the seed into the machine.
The two major disadvantages of the spiral separator are lack of flexibility in adjustment and low capacities. The lack of adjustment prevents fine-tuning the machine in order to make precise separations. Separations may be made only between good seed and contaminants with gross differences in shape, density and roundness.
In 1970 an article was published in Mississippi Farm Research (Vol. 33, No. 4, April 1970) on the use of seed dams to improve the action of the spiral separator. This device used wooden strips fastened to the inner flights with clothes pins. It was reported that, without the seed dams, 1%-3% of the seed of purple moon flower (a contaminant) remained in the good seed. With the seed dams, 99.9% of the purple moon flower seed were removed. Loss of good seed was 3% with the strips versus 1.9% without the strips.
A different type of seed dam has been recommended and used by Purdue University seed technologists. These consist of magnetic strips which are placed on the inner flights. Action and performance of these dams are identical to those used by Mississippi State University.
Both the wooden and magnetic seed dams have serious shortcomings in operation. First, the angle of the seed dams relative to a line tangent to the edge of the inner flight is critcal. In installing, and adjusting the angle of the seed dams, the machine must be shut down. The actual adjustment procedure is difficult and time consuming because of the inaccessibility of the seed dams. Secondly, the dams, as used, rarely stay in place once they are positioned. This means that constant readjustment is required for optimum separation.